No Post on system with DDR2 533Mhz

Hi.

I hope this was not an already discussed topic...
Have an unsusual no post situation on my assembled system. Here's the startup config:

Chassis Antec SLK3800B
PSU Antec Truepower Trio 650
Mobo MSI K9n Sli Platinum
CPU AMD 3800 x2
RAM: Transcend 512MB DDR2
Graphics XFX nVidia 7600GS in Sli
Hauppauge TV Tuner
Creative xtremegamer Sound card
2 120GB Seagate SATA in Raid 0
Gigabyte dual layer DVD burner

when I started out, i had 1.5GB of Transcend in slots 1, 2, 3. Slot 4 was vacant. So I guess 1 and 3 enabled a dual channel. all 3 are the same type in every aspect. now i picked up another identical 512 chip to complete the banks. the system would not post. monitor has a green blinking light and nothing happens. when i removed the memory from bank 4 and restarted, system posts and detects 1.5 GB.
swapped memory from bank 3 with the new one and system posted.
placed the new one in bank 4 with 3 vacant and still posted with 1.5 GB being detected.
after a bit of swapping all 4 memory modules in all 4 slots each and in every combo, what i found was that the system just does not post with all 4 banks filled.
is this a mobo memory controller failure? what am i looking at? system works like a breeze with the regular setup.
bios is updated to 1.8 from MSI, its the latest version. NO OVERCLOCKING DONE AT ALL. am I missing something here?

unlike in the other posts, here the system does receive power, fans spin, lights come on and all.
one feature in this mobo is that with usb devices plugged in, while posting, would beep each time corresponding to the number of usb devices plugged in. when the system has all memory banks filled in, no beeps at all and nothing on screen.

 

Do a mem test with just the new stick of ram installed, taking bets it will have errors. ed

 

Sorry. Done that already using Memtest and the memory diagnostics that comes on the Vista DVD.
system does post with the new memory and goes into the OS too.
on very rare occasion, i get a error during post of Memory H/W error and press F1 with 1.5GB RAM. on reboot, nothing wrong, system boots with 1.5GB. wont post with all banks occupied.

on this or these boards, memory controller is on the chipset itself right?